Amplitude modulation is used (with or without the addition of phase modulation) in a wide variety of communications systems such as mobile phones, cellular base stations for mobile phones and satellite communications.
It is often necessary to amplify amplitude and potentially also phase modulated RF signals. Conventional amplifiers have a generally linear relationship between input power and output power, during normal operation. As input power increases, efficiency and output power both increase. This means that high input power, leading to compression at the output, is necessary for high efficiency to be achieved. Eventually, the amplifier is driven into saturation where the relationship between input power and output power is non-linear. In saturation, the output power does not increase with further increases of input power level.
It is conventionally undesirable to operate an amplifier in saturation because driving an amplifier in the non-linear region also increases distortion of the signal. This means that when amplifying RF signals with high input peak to average power ratio, conventional amplifiers cannot be used in the high-efficiency region, they have to be used in their linear region, where efficiency is much lower.
One option to overcome this is to use outphasing amplifiers, such as Chireix amplifiers. In an outphasing amplifier, the amplitude part of an amplitude (and phase) modulated input signal is converted into two phase modulated signals, with constant amplitude envelopes. The phase modulated signals include the phase of the original signal as a common part, and additional antiphase components that, when recombination occurs, recreate the amplitude content of the original signal. The phase modulated signals are amplified by a pair of branch amplifiers, and combined to provide an amplified amplitude modulated output signal. Because of the constant amplitude envelope of the phase modulated signals passed to the branch amplifiers, the branch amplifiers can be operated at saturation, resulting in maximum efficiency.